The present invention relates to a mobile terminal apparatus used in, for example, a cellular radio communication system and, more particularly, to a mobile terminal apparatus using CDMA (Code Division Multiple Access) as a radio access scheme.
Recently, a mobile communication system using CDMA has attracted attention.
A CDMA mobile communication system uses spread spectrum communication and communicates, for example, as follows. A communication apparatus on the transmitting side modulates digital speech data or image data by digital modulation such as PSK modulation. The apparatus then converts the modulated data into a wideband baseband signal by using a spreading code. The apparatus converts the spread transmission signal into a signal having a radio frequency, and transmits it. A communication apparatus on the receiving side despreads the received radio frequency signal by using a code identical to the spreading code used by the communication apparatus on the transmitting side. This apparatus digitally demodulates the reception signal after this despreading by digital demodulation such as PSK demodulation, thereby reconstructing reception data.
CDMA has, for example, the following advantages which neither FDMA (Frequency Division Multiple Access) nor TDMA (Time Division Multiple Access) has:
(1) The use of a spread spectrum technique makes it possible to maintain high communication quality with respect to changes in communication environment such as fading.
(2) The use of a RAKE reception scheme makes it possible to perform soft handover and realize stable handover without causing any short break.
(3) Since many users share one radio frequency signal, a high frequency utilization efficiency can be attained.
In a CDMA mobile terminal apparatus, however, power consumption is generally high, and the battery life is short as compared with a mobile terminal apparatus using FDMA or TDMA.
In a cellular radio communication system, so-called handover is performed to switch one radio channel to another radio channel with which synchronization is to be established upon movement of a mobile terminal apparatus. In the handover, the mobile terminal apparatus detects the reception quality of a signal arriving from a base station, and determines on the basis of the detection result whether to perform switching. When the above reception quality is to be detected, a mobile terminal apparatus using FDMA or TDMA detects the reception electric field strength of a radio signal arriving from a base station. In contrast to this, a mobile terminal apparatus using CDMA must demodulate a radio signal arriving from a base station into a baseband signal first, and then perform digital processing for the signal. This is because, many mobile terminal apparatuses share one radio frequency (carrier) in CDMA, and digital signal processing including despreading processing for a spreading code must be performed to separate the respective channels.
For this reason, in the CDMA mobile terminal apparatus, many circuits in the reception system as well as the radio receiver operate in handover. As a consequence, a large current is consumed by the reception system.
In addition, in the CDMA mobile terminal apparatus, in the handover, the reception quality of a synchronized channel is compared with the reception qualities of other channels as synchronization establishment candidates to determine whether there is any channel whose reception quality is higher than that of the synchronized channel by a predetermined threshold or more. If a channel as a synchronization establishment candidate that meets this condition is found, the synchronized channel is switched to the channel of the synchronization establishment candidate.
In a conventional CDMA mobile terminal apparatus, however, the threshold for determining the reception quality is fixed to a predetermined value regardless of the reception quality of a synchronized channel. Even if, therefore, the reception quality of a synchronized channel is relatively good, and handover is not essentially required, handover is unconditionally performed whenever a channel as a synchronization establishment candidate higher in reception quality than this synchronized channel is found. As a consequence, handover occurs frequently, and much power is consumed by the terminal apparatus every time handover is performed.
In addition, in the CDMA mobile terminal apparatus, every time handover is performed, the reception qualities of all neighboring channels associated with the new channel with which synchronization is established are measured. For this reason, great power is consumed for every handover. As a result, the continuous standby time is greatly shortened.
It is an object of the present invention to provide a CDMA mobile terminal apparatus which reduces power consumption by reducing unnecessary handover, thereby prolonging the battery life.
In order to achieve the above object, according to the first aspect of the present invention, there is provided a CDMA mobile terminal apparatus having a function of receiving signals respectively transmitted from a plurality of base stations, selecting an optimal signal from the signals, and establishing synchronization with the selected signal, characterized by comprising:
First detection means for detecting a reception quality of a first signal with which synchronization has been established, second detection means for detecting a reception quality of at least one second signal as a synchronization establishment candidate, means, having information representing a correspondence between the reception quality of the first signal and a plurality of threshold values which is set in advance to change in accordance with the reception quality, for obtaining, on the basis of the information, a threshold value corresponding to the reception quality of the first signal which is detected by the first detection means, determination means for determining whether the reception quality of the second signal, detected by the second detection means, is higher than the reception quality of the first signal, detected by the first detection means, by not less than the obtained threshold value, and switching control means for performing processing for switching a synchronization establishment target signal from the first signal to the second signal only when the determination means determines that the reception quality of the second signal is higher than the reception quality of the first signal by not less than the obtained threshold value.
The second aspect of the present invention is characterized by comprising first detection means for detecting a reception quality of a first signal with which synchronization has been established, second detection means for detecting a reception quality of at least one second signal as a synchronization establishment candidate, quality classification means for classifying the reception quality of the first signal, detected by the first detection means, according to a plurality of different predetermined quality classes, a memory for storing predetermined threshold values in correspondence with the respective quality classes, selection means for selecting a threshold value corresponding to the reception quality of the first signal, detected by the first detection means, from the memory, determination means for determining whether the reception quality of the second signal, detected by the second detection means, is higher than the reception quality of the first signal, detected by the first detection means, by not less than the threshold value selected by the selection means, and switching control means for performing processing for switching a synchronization establishment target from the first signal to the second signal only when the determination means determines that the reception quality of the second signal is higher than the reception quality of the first signal by not less than the selected threshold value.
According to the first and second aspects of the present invention, different determination conditions are set in accordance with the reception quality of the first signal with which synchronization is being established, and the necessity for handover is determined on the basis of the determination condition.
If, for example, the reception quality of the first signal with which synchronization is being established is so high that reception can be performed without any error, the necessity to perform handover from the first signal to another signal is low. For this reason, no handover is performed unless a second signal sufficiently higher in reception quality than the first signal is detected.
In contrast to this, if the reception quality of the first signal with which synchronization is being established is too low to be allowed, it is necessary to perform handover from the first signal to another signal. For this reason, if a second signal higher in reception quality than the first signal is available, handover to the second signal is performed regardless of how much higher the second signal is in reception quality.
With this arrangement, if the necessity for handover is low, handover is not easily performed. This reduces the power consumption upon handover, and hence makes it possible to prolong the battery life. In contrast to this, if the necessity for handover is high, handover is easily performed. This makes it possible to maintain high-quality communication.
In addition, according to another characteristic feature of the first and second aspects of the present invention, if there are a plurality of second signals as synchronization establishment candidates, some signals are selected from the plurality of second signals, and the reception qualities of only the selected signals are detected.
With this arrangement, signals as reception quality detection targets are thinned out. Therefore, the power consumption required for reception quality detection processing can be reduced as compared with the case wherein the reception qualities of all signals are always detected. This makes it possible to prolong the battery life.
Furthermore, history information about the reception quality of each of a plurality of second signals as synchronization establishment candidates is managed. In thinning out the second signals, signals with high possibilities of providing good reception qualities in the future are selected on the basis of the above history information.
With this arrangement, the reception qualities of signals as promising synchronization establishment candidates can be detected. Therefore, although the reception qualities of only some signals are detected, this makes it possible to reduce the possibility of the inconvenience of missing a signal with a high reception quality.
In order to achieve the above object, a mobile terminal apparatus according to the third aspect of the present invention has the following arrangement.
Immediately after handover from the first signal to the second signal, the reception quality of the second signal with which synchronization has been newly established is very much likely to be high. For this reason, the necessity for handover from the second signal to another signal is low, and hence processing for checking the necessity for re-handover, i.e., re-maintenance processing, is generally unnecessary.
The present invention has been made in consideration of this point, and comprises a mode shift control means in addition to a handover control means. This mode shift control means omits the checking processing of determining the necessity to switch the synchronization establishment candidate from the second signal to another signal after synchronization is established with the second signal by the above handover processing.
With this arrangement, after handover is completed, the re-maintenance processing for a signal with which synchronization is established by the handover processing is omitted, and the mobile terminal apparatus directly shifts to the intermittent reception mode. This makes it possible to reduce the power consumption as compared with the case wherein re-maintenance processing is unconditionally executed after handover. Hence, the battery life can be prolonged.
According to the third aspect of the present invention, in the mode shift control means, after synchronization is established with the second signal by the handover control means, it is determined whether the reception quality of the second signal, detected by the quality detection means, is equal to or higher than a predetermined value. If it is determined that the reception quality of the second signal is equal to or higher than the predetermined value, the re-maintenance processing for the second signal is omitted, and a shift to the intermittent reception mode is made. In contrast to this, if it is determined that the reception quality of the second signal is less than the predetermined value, the above re-maintenance processing is executed for the second signal.
With this arrangement, even immediately after handover, if the reception quality of the second signal, with which synchronization has been newly established, has deteriorated, re-maintenance processing for the second signal is performed. If, therefore, re-handover from the second signal to another signal is required, handover can be immediately executed.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.